Reading devices or readers and an RFID transponder form a transmitter-receiver system for the identification and localization of objects which are attached to the RFID transponder. An RFID system thus includes an RFID transponder and a reader. RFID systems are used in a wide variety of technical solutions. For example, they are used in retail outlets on cash tills for identifying products to be paid for, or in logistics and in warehouses for the correct recording of incoming and outgoing goods, as well as for inventories and the retrieval of objects. Additional, they can also be used for identifying persons or also for identifying motor vehicles.
In motor vehicles, RFID systems are also deployed for the wireless, radio-based unlocking of doors of the vehicle and for the wireless, radio-base starting of a drive motor of the vehicle. If the corresponding RFID transponder is located within the transmission range of the reading device or reader, automatic starting or start/drive clearance of the drive motor or automatic unlocking of the vehicle doors takes place. The unlocking or starting takes place if the key with the corresponding RFID transponder is located within the transmission range of the reading device or reader. However, by amplification or a transmission system (data transmission system) of the transmission signal emitted by the reader, the key with the corresponding RFID transponder can also be inadvertently or unintentionally activated by thieves. If, for example, the key with the RFID transponder is in a house and the motor vehicle is in parking area in front of the house, through this amplification/transmission, the motor vehicle can be unlocked and started even though the RFID transponder is in the possession of the owner of the vehicle. Through this theft of the vehicle is possible.
U.S. Pat. No. 7,903,022 B2 describes a device for determining a distance between a transmitter and a receiver is known. The distance is determined by a phase change between a first signal and a second signal.